1. Field of the Invention:
This invention relates to a hydraulic cylinder apparatus and, more particularly, to a hydraulic cylinder apparatus suited as a precision servo.
2. Description of the Prior Art:
An actuator as a hydraulic cylinder apparatus is disclosed in Japanese patent publication No. 61-12125, for example. This actuator is provided with a cylinder having outside and inside cylinder sections extending coaxially and subjected to contraction pressure in the outside cylinder section and extension pressure in the inside cylinder section and a cylindrical piston rod having one end provided with a piston sliding in the outside cylinder section and the other end closed by a plunger. Since the pressure receiving area of the piston is formed equally to that of the plunger, the force generated by the piston at the time of contraction equals that generated by the plunger at the time of extension when equal fluid pressures act on the piston and plunger respectively. As a result, control quantity for controlling loads by means of the piston and plunger does not need to be changed to improve the operating property.
In the actuator according to the proposal as noted above, seal members disposed between the outside cylinder section and the piston and between the inside cylinder section and the piston rod or the like are constituted so as to prevent liquid from leakage, so that frictional force is increased as hydraulic pressure is increased, thus resulting in difficulty in using the actuator as a precision actuator.
The frictional force may be selectively reduced by a hydraulic cylinder apparatus (Japanese Utility Model Public Disclosure (KOKAI) No. 60-67404) which includes a seal member capable of leaking liquid and disposed between a cylinder and a piston therefor recovering leaked oil to a reservoir tank through a switch valve. However, when the reservoir tank is positioned above the cylinder apparatus, for example, a pump or the like for recovering the leaked oil to the reservoir tank is needed and this makes the cylinder apparatus complicated and expensive.